1. Field of the Invention
The present invention concerns data transmission, and is particularly, though not exclusively, concerned with the transmission of multiplexed data over a ring, the data having been created in accordance with the Synchronous Digital Hierarchy (SDH) as described in CCITT Recommendations G707, G708 and G709. The description applies equally to the CCITT SDH standard and to the N.American SONET standard; these are closely-related variants of synchronous multiplexing, differing somewhat in terminology.
2. Description of the Related Art
One of the advantages of a ring is the possibility it provides for geographical diversity to overcome a break in the transmission path, allowing transmission to occur via either of two routes around the ring. Protection can be provided by sending traffic in either direction and selecting from the appropriate direction at the destination receiver.
This selection may be by management command or may occur automatically after a break is detected. In either case, transmission may be in one direction at a time or in both directions simultaneously. The latter has the advantage that the receiver does not need to delay protection while it exchanges messages for control purposes with the transmitter or with a common management centre, since a simple changeover switch at the receiver can be triggered by the detection of a corruption on the initially received traffic.
In this respect, the protection described is similar to the well known "twinpath" or "I +1" protection, commonly used for the protection of line and radio systems.